Lonewolf 50
If you look at it from the perspective of Team Air France, the flight failed to do as intended: deliver plane and pax to Paris. Would a more useful form of that question be: "where were the seeds of mission failure sown?"
I suggest that's the driving question behind BEA's difficult task.
Not all seeds are immediately obvious. Nearly impossible to disprove, parts of the system soft/firmware might have been altered either by mistake or or purpose, maybe with the deliberate intention of causing this crash. Now that systems behave according to rules laid down in software they must be a prime candidate for interference by those who intend harm. Perhaps it hasn't yet happened somewhere, but for certain it will.
Oh, yes, it HAS happened. Erebus. TE901 November 1979. And, as Gordon Vette pointed out, there were a number of factors that came together to turn an incident into a tragedy.